Recent Advances in Polyoxometalates for Applications in Electrocatalytic Hydrogen Evolution Reaction

Hydrogen (H-2 ), a clean and sustainable energy carrier, is regarded as one of the most promising alternatives to carbon-based fuels. Hydrogen can be generated in a more sustainable way from renewable energy sources via electrocatalytic water splitting. However, the high cost and low abundance of the benchmarking platinum-based hydrogen evolution reaction (HER) catalysts hinder their widespread applications. Thus, developing highly efficient, stable, and low-cost electrocatalysts to replace platinum for HER is imperative, but remains a challenging task. Recently, efforts have been devoted to developing non-noble HER electrocatalysts, including transition metal carbides, oxides, phosphides, and sulfides. However, traditional synthetic strategies cannot effectively control active sites and the catalysts tend to aggregate under high temperature. Recently, polyoxometalates (POMs) have been applied as precursors for the preparation of non-noble HER electrocatalysts as they contain discrete metal-oxygen clusters with well-defined structures. POMs are typically composed of oxygen ligands and high-valent metal ions such as V(V), Mo(VI), and W(VI), which can serve as V, Mo, and W sources to produce the corresponding metal carbides, oxides, phosphides, and sulfides by pyrolysis at high temperature. Some POMs may also contain a series of redox-active heteroatoms, which are usually named hetero-polyoxometalates. These can serve as precursors to electrocatalysts with uniform heteroatom doping. Moreover, direct applications of POMs as molecular catalysts in HER have, in recent years, received rapidly growing attention. This is because POMs not only serve as mediators or molecular catalysts to facilitate the HER, but can also be deposited on the electrode surface to catalyze the HER. However, the interpretation that HER catalytic activity enhancement is due to the intrinsic catalytic properties of the electrodeposited polyoxometalate or the deposition of small amounts of platinum has been highly debated. Reviewing these studies may help us understand the intrinsic active sites as well the intrinsic HER mechanism of POMs and POMs-derived catalysts, and thus design more efficient HER catalysts. This review, therefore, focuses on recent progress in the applications of POMs and their derivatives in electrocatalytic HER. Firstly, basic HER mechanisms for common metal catalysts and POMs molecular catalysts are discussed along with challenges in the field of HER. Next, applications of POMs molecular catalysts and POMs-derived catalysts in HER are summarized. Finally, some perspectives of POMs-based catalysts/pre-catalysts for electrocatalytic HER are proposed.